In a cable system, a network of interconnected electrical cables, referred to as a cable plant, is commonly used to deliver information to subscribers. A cable plant enables a broadband transmission of signals, such as television signals, from a head end facility to a multitude of home receivers. A broadband coaxial cable is advantageously used in this application because it supports a wide frequency bandwidth and provides signal shielding at a moderate cost in comparison to other media. The wide frequency bandwidth permits definition of a substantial number of information channels on the coaxial cable thus allowing simultaneous broadcasting of many channels.
Cable systems have, in recent years, moved beyond merely broadcasting analog television signals over the cable to subscribers in their homes. Digital video services have become more popular than analog television services due to more efficient bandwidth utilization, and due to their intrinsic high-definition video (HDTV) capabilities. Further, a subscriber of a community antenna television (CATV) network nowadays has a transceiver, or a modem, which allows the transmission of digital signals upstream to the head end of the network. Among many services the subscribers have access to by having the transceiver are: an Internet service, a home shopping service using a television catalogue, and a voice-over-IP phone service.
In bidirectional cable networks, the upstream and the downstream signals occupy separate frequency bands called upstream and downstream spectral bands. In the United States, the downstream spectral band typically spans from 50 MHz to 860 MHz, while the upstream spectral band spans from 5 MHz to 42 MHz. Many downstream information channel signals, each channel occupying a separate 6 MHz sub-band, co-propagate in the downstream spectral band, and many upstream signals co-propagate in the upstream spectral band. The frequency separation of the upstream and the downstream signals allows bidirectional amplification of these signals propagating in a common cable in opposite directions.
The increased cable bandwidth utilization and the bidirectional use of cable plants have increased sensitivity of cable networks to network impairments. One such impairment, affecting mostly upstream signals, is common path distortion (CPD). Although CPD varies in severity and manifests itself in many different ways, it has a very distinctive spectral signature. Typically, CPD is characterized by a significant rise of the noise floor across the upstream spectral band. The rise of the noise floor is accompanied by spectral beats spaced apart at 6 MHz intervals. The spectral beats also occur in the upstream spectral band. CPD can cause a major reduction of carrier-to-impairment power ratios, leading to errors in upstream digital transmissions.
CPD is a signal distortion due to a nonlinear response of an element disposed in a common path of a bidirectional cable network. The common path means a path shared by the upstream and the downstream signals propagating in the network. It is well known that a sinusoidal signal at a single frequency, upon propagating through a component or a module having a nonlinear transfer function, gives rise to so called harmonics, or signals at multiples of the signal frequency. When two single-frequency signals co-propagate through such a nonlinear component or a module, signals at a differential, or “beat” frequency and at a sum frequency also appear, in addition to the harmonics. A term “frequency mixing” is sometimes used to describe these nonlinear phenomena. Due to the frequency mixing, signals in the upstream spectral band give rise to spurious noise in the downstream spectral band, and vice versa, resulting in a rise of a noise floor in both spectral bands. In practice, the rise of a noise floor in the upstream spectral band is much more pronounced than in the downstream spectral band because a signal in the downstream spectral band has a much higher total power and a much broader spectral content than a signal in the upstream spectral band.
One well-known source of nonlinearity is a radio-frequency (RF) amplifier used to amplify signals in a cable system. Fortunately, individual RF amplifier modules are unidirectional and therefore are not disposed in the common path of a cable plant. Even when the frequency mixing takes place in an RF amplifier, the generated harmonics and frequency beats are filtered out by diplex filters used to separate the upstream and the downstream signals counter-propagating in the cable plant. Another source of nonlinearity is a regular connector used to connect two or more cables together. A thin metal oxide layer, gradually developing on a surface of contacting metal parts of the connector, acts as a diode, and because a diode is a nonlinear device, the oxidized connector becomes a source of nonlinearity capable of mixing frequencies of signals propagating therethrough. If the oxidized connector is disposed in the common path of the upstream and the downstream signals, it becomes a source of CPD. Many hundreds and even thousands of connectors are typically installed in a cable plant. Some are installed in areas that are not completely weather-proof, which facilitates metal oxidation; some are installed in subscribers' premises, which are not readily accessible. Multitude and limited accessibility of connectors and other network components and modules make a task of locating a CPD source particularly difficult.
The problem of locating a CPD source in a cable system has been recognized in the art. The prior-art approaches can be broken down into two categories. In approaches of the first category, an active probing signal is generated at a head end facility of a cable system, and an “echo” signal having a CPD specific spectral signature is detected. A distance to a CPD source is then determined from the arrival time of the “echo” signal. In approaches of the second category, a CPD signal due to a pre-selected pair of downstream channel signals is simulated at the head end, and the result of simulation is correlated with a signal in the upstream spectral band having the upstream signals filtered out. A position of the correlation peak detected is indicative of a distance to a source of CPD.
An approach of the first category is taught by Eastment in a PCT Application WO2000057571 incorporated herein by reference. Referring to FIG. 1A, a frequency diagram is presented showing a downstream spectral band 10 and an upstream spectral band 11, downstream carrier signals 12, an active probe signal consisting of modulated signals 13 and 14 having higher frequencies than the frequencies of the downstream carrier signals 12, and an “echo” CPD signal 15 at a beat frequency between the signals 13 and 14. In operation, the modulated signals 13 and 14 are injected into the downstream path of the cable network, and the CPD signal 15 is detected in the upstream spectral band 11. The CPD signal 15 is correlated with the signals 13 and 14, so as to determine the time delay associated with one or more CPD sources of the cable network. A distance to a CPD source is then calculated from the determined time delay of the CPD signal 15 relative to the modulated signals 13 and 14.
An approach of the second category is taught by Zinevitch in US Patent Application 20060248564 A1, incorporated herein by reference. Referring to FIG. 1B, a frequency diagram is presented showing the downstream spectral band 10 spanning from 50 MHz to 860 MHz and the upstream spectral band 11 spanning from 5 MHz to 50 MHz, non-conterminous downstream channel signals 16 and 17, and a CPD signal 18. Central frequencies of the downstream channel signals 16 and 17 are separated by ΔF. The CPD signal 18 is a second-order nonlinear product of the downstream channel signals 16 and 17 and, therefore, it has a central frequency at ΔF. In operation, a second-order CPD signal is calculated at the head end of the cable network, and the upstream signal is digitized and correlated with the calculated second-order CPD signal. A peak in the correlation function indicates presence of the second-order CPD impairment in the cable network. The position of the peak is used to calculate a cable length to the CPD impairment source.
The prior-art approaches suffer from a number of drawbacks. In approaches of the first category, the central frequencies of the active probing signals, for example the modulated signals 13 and 14 in FIG. 1A, have to be carefully selected so as not to overlap with the existing downstream channel frequencies to avoid signal interference. Furthermore, the CPD signal 15 of FIG. 1A has to be at a frequency not already occupied at the moment of the measurement by existing upstream signals, otherwise the upstream signal transmission can be disrupted. Disadvantageously, the approaches of both categories require complicated electronic equipment for analog and digital processing of modulated RF signals. For example, in a device taught by Zinevitch, a complicated adaptive filter is provided for filtering out the upstream channel signals, and a digital signal processor is provided for calculating the correlation function.
Accordingly, it is a goal of the present invention to provide an apparatus and a method for detecting nonlinearity in a cable plant and determining a cable length to a source of the nonlinearity, that is simple, inexpensive, and does not require probe signals to be injected into the network.